Computing systems typically utilize one or more display monitors to provide a visual input/output capability for the system. Since display monitors are similar in many respects to conventional television receiver displays, many technologies, including the present invention, may be applied effectively to both. In both systems, a cathode ray tube (CRT) includes an evacuated envelope usually made of high-strength glass. The envelope includes a generally flat or slightly curved faceplate or viewing screen together with a funnel shaped bell and extending neck. The interior side of the faceplate supports a phosphor screen. In monochrome displays, a single electron gun is supported within the CRT neck and is directed toward the phosphor screen. The electron gun produces a beam of electrons which are directed toward the faceplate striking the phosphor screen and causing visible light to be emitted therefrom. In color display systems, a plurality of electron guns are used together with a phosphor screen which supports plural areas of phosphors having differing color light emitting characteristics. A shadow mask or similar structure is interposed between the electron guns and the phosphor screen to cause each of the electron guns to stimulate an associated type of colored light emitting phosphor.
Whether the display system is monochrome or color, the electrons emanating from the electron gun or guns form a CRT beam which is scanned in both the horizontal and vertical directions across the faceplate to form a raster. In most instances, the horizontal scan system is operative at a higher frequency than the vertical scan system. Thus, the horizontal scan moves the electron beam rapidly from side to side across the faceplate while the vertical scan system causes the successive horizontal scans to be moved progressively from top to bottom to complete a display frame and form the raster.
In the majority of the presently used display systems, electron beam scanning is accomplished by electromagnetic deflection of the CRT beam. A deflection yoke is supported upon the CRT envelope between the electron guns and the faceplate. The deflection yoke supports a plurality of deflection coils which are coupled to the horizontal and vertical scan systems. Horizontal and vertical scan signals provided by the respective scan systems are coupled to the windings of the deflection yoke to produce corresponding electromagnetic fields which bend the electron beam and thereby direct it to the desired portion of the CRT faceplate. Both the horizontal and vertical scan signals include longer duration sloped scan portions followed by shorter duration high amplitude retrace portions. The latter are utilized at the completion of each respective scan interval to return the electron beam to its starting position. In addition, the retrace portion of the horizontal scan signal can be used to develop the high voltage necessary to accelerate the electron beam toward the CRT faceplate.
The character of the image displayed in a CRT display system results from variation or modulation of the intensity of the scanned CRT electron beam. This intensity modulation must be properly timed or synchronized to the horizontal and vertical rate scanning of the raster. Thus, as the electron beam is scanned across the faceplate to form a raster, the desired portions of the faceplate are illuminated by synchronized modulation of the electron beam to provide the desired image.
One of the problems associated with CRT displays which is also shared by television receivers arises from the relationship between CRT beam current intensity, high voltage potential, and deflection sensitivity. As CRT beam current intensity changes, the loading imposed upon the high voltage power system is also changed which in turn causes an inverse change in the high voltage potential. As high voltage potential is changed, the degree of electron beam bending which results from the electromagnetic fields of the deflection yoke (deflection sensitivity) is also changed. For example, an increase in CRT beam current imposes a greater load upon the high voltage system causing a reduction of high voltage potential. The reduction of high voltage potential produces a corresponding increase in deflection sensitivity (more electron beam bending) which in turn causes the raster to be enlarged or "bloom". Because the CRT beam current intensity modulation is synchronized to the horizontal and vertical scan, the displayed image is undesirably enlarged as the raster blooms.
In the event the changes in beam current exist for relatively long time intervals (eg. several vertical scan periods), the entire raster becomes expanded and the displayed image is correspondingly enlarged. While this general enlargement of the display may be annoying, an even more deleterious effect results from abrupt relatively short term changes in beam current. Such shorter duration beam current changes cause localized or partial blooming and display enlargement. The result is that portions of the displayed image are expanded or shifted with respect to the remaining image elements.
To meet the difficulties associated with raster blooming in response to beam current changes, designers of computer display monitors and television receivers have usually attempted to "stiffen" or regulate the high voltage generating system and render it less sensitive to beam current changes. The detailed structure of such systems varies substantially. However, all generally include some means for sensing CRT high voltage and regulating the high voltage potential in a compensating manner. In some systems, a high voltage shunt regulator is used while in others the retrace time is altered. In addition, completely redundant horizontal scan systems and high voltage systems are used. The objective is to increase high voltage power as beam current increases, thereby maintaining a relatively constant high voltage. To the extent the high voltage potential is maintained constant, the above-described changes in deflection sensitivity in response to beam current and the resulting raster blooming are avoided. However, shunt regulators are expensive and waste power and redundant systems are very costly. Changing retrace time is complex due to interactive effect upon the remainder of the system.
In addition to the above-described circumstances which render cost efficient and effective high voltage regulation difficult in both television receivers and display monitors, additional problems arise which are particularly evident in many display monitor environments. While the horizontal scan and high voltage generating systems of television receivers are designed to meet narrow frequency variations, display monitors for computing systems are often required to operate over a substantial frequency range. Because the horizontal deflection and high voltage systems of display monitors include a plurality of complex tuned highly interactive system components, variation of scan frequency upsets the relationship between horizontal deflection and high voltage generation. In particular, a difficult problem arises in that deflection yoke power and high voltage system power do not properly track during frequency variation if display size is maintained constant. Thus, the problem of high voltage regulation in display monitors is further exacerbated in multiple frequency environments.
While the above-described regulation systems provide some benefit in responding to the problem of raster blooming, they tend to be complex, expensive and/or inefficient and are often unable to meet the demands of a multiple frequency environment.
There remains, therefore, a need in the art for an inexpensive, efficient high voltage regulation system which effectively controls high voltage, compensates for changes in CRT beam current intensity and accommodates substantial changes in scan frequency.
Accordingly, it is a general object of the present invention to provide an improved high voltage regulation system for CRT displays. It is a more particular object of the present invention to provide an improved high voltage regulation system for CRT displays which substantially maintains image size despite changes an image intensity and scan frequency. It is a still more particular object of the present invention to provide an improved high voltage regulation system for displays which is efficient and relatively inexpensive.